1. Field of the Invention
The present invention relates to a mobile communications system, and more particularly to a method for allocating common packet channels in a next generation mobile communication system.
2. Background of the Related Art
FIG. 1 shows a transmission structure of a related art common packet channel (CPCH). Referring to FIG. 1, a related art CPCH includes a CPCH status indicator channel (CSICH) for transmitting status information of a CPCH of a current cell from a base station and, an access preamble (AP), which is transmitted by a mobile station that received the CSICH in order to request allocation of a specific CPCH. An AP acquisition indicator channel (AP-AICH) is provided for transmitting a response signal to the AP by the base station, and a collision detection preamble (CD-P) is provided for detecting a collision of the CPCH generated when a plurality of mobile stations request the same CPCH. The CD-P thereafter releases the collision. Next, a collision detection preamble acquisition indicator channel (CD-ICH) is provided for transmitting from the base station a response to the CD-P, and a power control preamble (PC-P), having a 0 or 8 slot length, is provided for setting transmission power level before transmission of a message part. The CPCH also includes a down link-dedicated physical control channel (DL-DPCCH) for implementing closed loop power control (CL-PC), and a message part for transmitting user packet data. The message part includes a data part and a control part.
A related art procedure for transmitting the CPCH will next be described.
First, a mobile station which desires to transmit packet data identifies a currently available (or non-available) channel by referring to a CSICH being broadcasted from a base station. The mobile station tries to access the base station when a CPCH that can support a desired transmission data rate is vacant.
The mobile station then transmits an AP to the base station to inform a desired CPCH. That is, the mobile station respectively selects an AP signature and an access slot, and transmits an AP consisting of the selected AP signature and access slot to the base station in compliance with a start point of the access slot. The AP signature indicates each CPCH.
Thereafter, the mobile station increases transmission power to retransmit the AP in compliance with the start point of the access slot when an acquisition response of the AP is not received after a prescribed time period. Such retransmission is repeated for a number of threshold times.
The base station receives the AP from the mobile station to sense a maximum data rate or a minimum spreading factor requested by the mobile station. Then, the base station determines whether to allocate a CPCH requested by the mobile station after considering resource usage of the current CPCH and an amount of total traffic.
Meanwhile, in an optical band code division multiplexing access communication system, since one cell can serve up to 16 CPCHs or less, 16 signatures exist. The mobile station selects one of the 16 signatures to transmit it to the base station.
At this time, if a minimum spreading factor SFmim of a CPCH serviced by a cell is below 32, the 16 signatures are one-to-one mapped with a node having a spreading factor of 16 in a channelization orthogonal variable spreading factor (OVSF) code tree, and then transmitted (see FIG. 2, for example). This means that the 16 APs are respectively mapped with a channelization code of a message part of the CPCH one to one, and respectively indicate 16 CPCHs.
Subsequently, the base station determines whether the CPCH can be allocated. If the CPCH can be allocated, the base station transmits the signature equal to the received AP as an acknowledgment (ACK) signal in compliance with the start point of the access slot. If the CPCH cannot be allocated, the base station transmits an inverted signature of the received AP as a non-acknowledgment (NACK) in compliance with the start point of the access slot.
When a number of mobile stations simultaneously transmit an AP with the same signature, the base station does not identify the mobile stations from the same signature, and therefore transmits the ACK signal to all mobile stations. Thus, the mobile stations that received the ACK signal transmit a CD-P to the base station to detect a collision. Any one of the 16 signatures equal to the AP is used as the CD-P. Likewise, any one of 16 signatures is used as scrambling codes in the same manner as the AP, but codes shifted as much as 4096 chips are used as the scrambling codes.
When the base station receives one CD-P, the base station determines that a collision has not occurred, and transmits the signature equal to that of the received CD-P to the mobile station through the CD-ICH. However, when the base station receives a number of CD-Ps, the base station determines that a collision has occurred, and selects the CD-P having the highest power among the received CD-Ps, and transmits the CD-ICH to the corresponding mobile station.
Furthermore, the base station transmits a CA-ICH containing channel information in a signature format to the mobile station. At this time, the channel information contained in the CA-ICH includes a channelization code and a scrambling code of a down link-dedicated physical control channel (DL-DPCCH) and a physical common packet channel (PCPCH). The CD-ICH and the CA-ICH are simultaneously transmitted to the mobile station.
Afterwards, the mobile station that received the CD-ICH and the CA-ICH starts to transmit a message. The mobile station controls transmitting power using the PC-P, if necessary, to start to transmit the message consisting of a data part and a control part. Meanwhile, the base station transmits the DL-DPCCH to the mobile station. The mobile station transmits the PC-P for a constant time of 0 or 8 slots through the signature information of the CA-ICH before transmitting the message consisting of the data part and the control part through a physical channel.
Meanwhile, in transmission of the PCPCH, the mobile station uses a code mapped one-to-one with the signature of the CA-ICH as a scrambling code, and also uses a node having a spreading factor of 2C (2.0) on an OVSF code tree against all of CA-ICHs as a channelization code.
Meanwhile, the signatures of the AP are one-to-one mapped with the channelization codes of the message part. That is, as shown in FIG. 3, if the minimum spreading factor SFmin of the data part is 32, in the OVSF code tree, the AP selects one of codes having spreading factors of 32˜256 in an up branch direction from the node having a spreading factor of 16 as a channelization code Cd of the data part to map therewith. It also selects a code located last in a down branch, i.e., a code having a spreading factor of 256, as a channelization code Cc to map therewith. A gold code, a M-sequence code, and a Kasami code may be used as a scrambling code of the message part.
Therefore, when the base station receives the message part mapped as above, the base station decodes the control part using the channelization code Cc determined by the AP signature. Since the data part corresponds to one of spreading factors 32˜256, the base station partially codes an OVSF code having a spreading factor of 16 and then decodes the control part. The base station detects an exact spreading factor of the data part to decode the data part.
Meanwhile, if the spreading factor serviced by the cell is below 16, i.e., if the minimum spreading factor SFmin is 4, 8, and 16, the AP signature is not mapped with the channelization code of the message part one to one. Accordingly, as shown in FIG. 4, a fixed channel structure is used.
FIG. 4 shows a mapping structure between the AP signature and the CPCH when a related art spreading factor is 4. Referring to FIG. 4, if the minimum spreading factor SFmin of the CPCH serviced by the cell is 4, 8, and 16, a specific AP is fixed to indicate a specific channel.
That is, AP#0˜AP#7 of the 16 APs indicate CH#0, AP#8˜AP#9 indicate CH#1, AP#10˜AP#11 indicate CH#2, and AP#12˜AP#15 respectively indicate CH#3˜CH#6. At this time, in CH#0˜CH#2, the channelization code Cd of the data part is determined as one of codes of the next node as shown in FIG. 5. The channelization code Cc of the control part is determined as a code located last in a down branch direction, i.e., a code having a spreading factor of 256. At this time, since the minimum spreading factor SFmin is below 32 in CH#3˜CH#6, partial coding is performed with a spreading factor of 16 as described above. In this fixed channel structure, CSICH denotes a status of CH#0˜CH#6.
Accordingly, when a specific mobile station desires to transmit data having a spreading factor of 4, one of AP#0˜AP#7 corresponding to CH#0 is selected as the signature of the AP. When a specific mobile station desires to transmit data having a spreading factor below 32, one of AP#12˜AP#15 is selected.
The aforementioned related art has several problems. For example, since 16 signatures of the CA-ICH are used in the related art CPCH, PCPCHs more than 16 cannot be allocated with the CA-ICH. In this case, when considering that the next generation mobile communication system should support 64 maximum PCPCHs per one cell for low rate data service, a method for allocating a CPCH is required to more efficiently use a channel resource.
Additionally, in the related art method for allocating a CPCH, a mapping method between a signature of the CA-ICH for channel allocation and an OVSF code that is a channelization code of the CPCH, has not been suggested. Accordingly, the channel resource is not efficiently used. For this reason, a mapping method between the signature of the CA-ICH and the OVSF code is required.
Further, in the related art transmission structure of a CPCH, particularly, in case of AP, the CPCH has a data rate of 960˜15 kbps and thus a spreading factor is variable at 4˜256. In this respect, there are several problems.
For example, the signature of the AP in access step of the mobile station is mapped with 16 nodes having a spreading factor of 16 on the OVSF code tree. Accordingly, transmitting data having a spreading factor below 32 can be accomplished. However, in transmitting data having a spreading factor of 4, 8 and 16, one-to-one mapping between the signature of the AP and the node is not performed. In this case, a fixed channel structure is required. Accordingly, to use a data rate of 4 in the fixed channel structure, the specific mobile station should select one of 8 signatures, as opposed to 16 signatures. This increases the probability of collision of the CPCH in the access step. At this time, if data having a spreading factor of 32 or less is transmitted, as shown in FIG. 4, the AP of one of AP#12˜AP#15 should be selected. This also results in that the probability of collision of the CPCH increases in the access step.
Next, if the specific mobile station uses CH#0 having a spreading factor of 4, AP#4˜AP#6 can be used for data transmission having a spreading factor of 32 or less. However, in the fixed channel structure, the APs remain vacant. This causes a problem that the channel resource is inefficiently used.
Moreover, since different spreading factors are supported to the respective AP signatures, a separate mapping table between the AP signature and the spreading factor is required. Also, since the system should periodically broadcast the mapping table information to the mobile station through a broadcasting channel BCH, problems arise in that the capacity of the system increases and interference increases.
The above references are incorporated by reference herein where appropriate for appropriate teachings of additional or alternative details, features and/or technical background.